1. Technical Field
This invention relates to relay devices for communication systems, and communication systems having such devices.
2. Related Art
In a normal cellular telephone system, a plurality of cellular telephones (also known as cell phones or mobile telephones) normally each communicate with a base station by sending and receiving radio signals to and from the base station.
The base station normally has an antenna mounted on a building or on the ground, whereas cellular telephones are portable and can easily be carried by a user when the user travels from one location to another location. However, cellular telephones can only normally transmit radio signals within a certain range. If the user travels too far from the base station, or to a region where there is an obstacle between the user's cellular telephone and the base station, the radio signal travelling between the base station and the cellular telephone may become too attenuated to reliably convey the original information. Consequently the radio signal will not be properly received. Unless there is another base station within the range of the cellular telephone, the user will not be able to communicate over the cellular telephone.
Networks are known which have a plurality of mobile nodes. A message is conveyed by passing the message from one node to the most adjacent node, and then to the next adjacent node, and so on until the message reaches its destination. Such networks are commonly referred to as “parasitic” networks. Such systems may have a fixed base station, although not all of them do. It is known, for example from United Kingdom Patent Specifications GB2326059 and GB2346511 to use one or more mobile terminals of a cellular radio system to relay traffic between other mobile terminals and the nearest base station, thus reducing the power required by the base station and allowing radio access to places not reachable directly from the base station.
Base stations are usually fitted with directional antennas, to concentrate their signal gain in the azimuthal direction where most mobile terminals are likely to be found, but there is still a falling off of signal strength with distance. However, as portable mobile terminals may be held in any orientation, they require omnidirectional antennas, in which signal strength falls off with distance according to the inverse square law. The use of mobile terminals as relays reduces the power required by the system as a whole. For example, if a hop of length x requires a given transmitter power strength P at the transmitter to generate a received strength E, to provide the same received signal strength E on a hop of length ×/3 requires a transmitter power of P/9. Thus, three hops of length ×/3 require a total power output of only 3P/9=P/3.
Users of such relay devices incorporated into mobile telephones would therefore benefit from reduced power consumption overall, since although power would be taken from their mobile device to relay calls between other users, they would benefit from reduced power consumption on their own call traffic to a much greater degree. However, most mobile devices are battery powered. The availability of reduced power consumption would be of little consolation to a user who has left his device on standby awaiting a call if, when the call comes through, his battery has already been drained by its use as a relay. It is an object of the present invention to address this issue.